The official website announced that Pokémon Go is now available in the United States on Android and iOS devices.
Note that at the moment the Pokémon Trainer Club account is down for maintenance so you can't login with it but the official Pokémon Go Twitter account confirmed that they are working on it.

Due to the incredible number of #PokemonGO downloads, some Trainers may experience sign-up difficulties. Don’t worry, our team is on it!